Ceramic structures (for example, honeycomb structures) are often used for filters and catalyst carriers, for example, in exhaust gas purifying systems of heat engines, such as internal combustion engines, and combustion equipment, such as boilers, in liquid or gaseous fuel reformers, and in water and sewage purifying systems. In particular, ceramic structures are suitably used in diesel particulate filters (hereinafter referred to as DPFs) or high-temperature gas dust collector for collecting and removing particulate matter from dust-containing fluid, such as exhaust gas emitted from diesel engines.
Such a ceramic structure traps and removes unnecessary particulate matters when fluid to be treated passes through the pores in porous partition wall of the structure, or the ceramic structure is allowed to carry a catalyst on the surfaces of the porous partition wall or in the pores so that the fluid is brought into contact with the catalyst.
If the ceramic structure has large holes penetrating the porous partition wall, the large holes negatively affect the filtration performance or the ability to function as a catalyst carrier of the ceramic structure, and bring about defects. It is therefore important to detect such large holes. It is also important to examine the size and number of the holes penetrating the partition wall, in estimating the ability if the ceramic structure is used as a filter or the like.
In order to inspect filters having such a structure for a defect, a method using powder having a specific particle size has been proposed in, for example, Japanese Unexamined Patent Application Publication No. 2000-193582. This method inspects pinholes in the partition walls by detecting the powder discharged from the filter with a particle counter. Unfortunately, the method allows the powder to remain in the filter, and the remaining powder must be removed. Also, this method is not suitable for inspecting the pore size of the filter because the powder cannot be discharged unless the filter has pinholes.
Another method (LS, light scattering) has also been proposed in, for example, PCT Publication No. WO 02/082035 for detecting a defect in a test body. In this method, fine particles are introduced into the test body and particles discharged from the test body are irradiated with laser light so that the particles are visible. This method is advantageous in identifying the position of the defect in the cross-sectional direction, but unsuitable for inspecting the shape and size of the defect.
For inspection for, particularly, an internal defect in a filter having the above structure, it has been necessary to destroy the test body or product since old days. Destructive inspection is much expensive in time and effort and less accurate. Specimens subjected to destructive inspection are not used for other inspections for evaluating the entirety of the product, such as strength, disadvantageously.
In order to overcome those disadvantages, nondestructive inspection with X rays is beginning to be applied to inspect structures for defects. For example, an industrial X-ray CT apparatus makes it possible to detect internal fractures or defects which have not been detected by conventional radioscopy and to analyze the three-dimensional structure of test bodies.
However, this method has been mainly approached from the viewpoint of increasing the voltage of the X-ray tube to enhance the spatial resolution (that is, how finely the apparatus can inspect a test body), but has not taken into account the density resolution (that is, for example, how finely the apparatus can show the difference in density among adjacent materials in a test body). Consequently, this method cannot sufficiently estimate the density of particularly ceramic structures, and thus makes unclear the boundary between the internal defect and the internal structure. Thus, it has been difficult to accurately identify the position, shape, and size of the internal defect.